pH is one of the most important chemical parameters for monitoring chemical and biological processes. It is commonly used, for example, in the food industry, minerals processing, bioprocessing and environment monitoring. pH is commonly measured by using glass pH electrodes. Glass pH electrodes have good sensitivity and stability. However, they suffer from a number of serious disadvantages, such as high impedance, mechanical fragility, instability in very acidic solutions and high temperatures, slow response and vulnerability to membrane fouling. For applications where the volume of solution is restricted, glass electrodes are not suitable due to the difficulties in miniaturization.
As a result, non-glass pH sensors, especially solid-state pH sensors using metal oxides, began to draw considerable attention, because they are robust and less sensitive to cation interference. Fog et al., Sensors and Actuators, 1984, 5, 137-148) describe metal oxide films formed on the surface of precious metal electrodes and their use in measuring hydrogen ion concentration. Electrode potentials due to the oxidation-reduction reaction of the metal oxides are dependent on the hydrogen ion concentration. The useful metal oxides include TiO2, RuO2, RhO2, SnO2, Ta2O5, OsO2, PdO2, PtO2, IrO2, and the like. The hydrogen ion selective electrodes using metal oxides are mostly based on the fact that the potentials due to the reversible oxidation-reduction reactions of the metal oxides are dependent on the hydrogen ion concentration. These metal/metal oxide electrodes exhibit a Nernstian or near-Nernstian response to pH. However, there are also several drawback compared to glass pH electrodes. Most significant ones are (i) interference caused by halogen anions, redox active species and complexing agents, (ii) drift and (iii) hysteresis.
Quan et al., Bull, Korean Chem, Soc. 2005, 26, 1585-1588) describes iridium oxide/carbon-polymer composite hydrogen ion electrodes. These composite electrodes are said to have an advantage in that they are composed of polymer materials and carbon black particles or graphite particles, which are conductors, and uniformly include iridium oxide particles, exhibiting selective sensitivity to hydrogen ion. The electrodes have hydrogen ion selectivity and physical stability due to the mechanical strength of the polymers, thereby easily obtaining a renewable electrode surface through a simple polishing process, whenever the electrodes are inactivated or contaminated.
Iridium oxide/carbon-polymer composite pH electrodes have problems in that, although the electrodes have improved physical stability and surface renewability compared to conventional glass electrodes or polymer film electrodes, the manufacturing method of the electrode is complicated, the pH dependency of the electrodes varies greatly depending on the electrodes, and hysteresis occurs during repeated use of the electrodes. U.S. Pat. No. 8,486,238 addresses the problems of the Quan et al. composite electrode by providing an iridium oxide glass or ceramic composite electrode which formed by sintering at a temperature of preferably 600° C. to 800° C. for 3 to 5 hours.
Miao et al., Sensors and Actuators B, 192 (2014)399-405 disclose a tantalum pentoxide based electrolyte-ion sensitive membrane-oxide-semiconductor (EIOS) pH sensor and studies possible interference from a range of metal ions in acid solutions.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of any of the claims.